Download Magma ocean influence on early atmosphere composition and mass

Goldschmidt 2012 Conference Abstracts
Magma ocean influence on early
atmosphere composition and mass
MARC M. HIRSCHMANN1* , HONGLUO ZHANG1
1Dept.
Earth Sci. U. Minneosta Minneapolis, MN USA
[email protected] (*presenting author)
The composition and mass of the atmosphere overlying early
terrestrial magma oceans (MOs) likely had a key influence on Earth’s
early thermal and dynamical evolution, its geochemical
differentiation, its path to an equable climate, and development of
prebiotic chemistry. It also set the initial conditions for development
of deep Earth volatile cycles. Key questions are how MO processes
partitions volatiles between the atmosphere, mantle, and core and
how reaction between the the MO and overlying vapor influences the
composition of the nascent atmosphere.
The availability of Fe alloy in a MO creates reducing conditions,
and the expectation that overlying atmospheres consist chiefly of H 2
and CO, but this may be incorrect if the mean depth of alloy-silicate
equilibration is deep, if convection in the MO homogenizes Fe3+/FeT
throughout the magma column, and if the effect of decreased
pressure on the chemical potentials of Fe2+ and Fe3+ in the magma
increases relative fO2. Low pressure data [1,2] suggest the opposite –
decreases in ∆fO2 on isochemical decompression – but Mössbauer
spectroscopy of andesite glasses quenched from 1-6 GPa
provisionally indicate that this relationship reverses at high pressure.
Consequently, it is likely that the overlying atmosphere consisted
chiefly of H2O+CO2, whilst reducing (~IW-1.5) conditions prevailed
deep in the MO.
Owing to low solubility of C-species in reduced magma,
virtually all of the available C in the MO and the atmosphere may be
partitioned into alloy, leaving a virtually C- free mantle and
exosphere. However, it is likely that the MO equilibrates with only a
small fraction of metal (<5% [3] and possibly <1%). As the alloy
cannot absorb ~ 8 wt.% C, excess reduced C may saturate as
diamond. This too will greatly limit the partial pressure of CO2 in the
overlying atmosphere. Floatation of diamond in the MO [4] and
could create the principle initial reservoir of C in the BSE.
Following or coinciding with segregation of metal (but not
diamond) to the core, the Moon-forming impact resets the volatile
distribution in the BSE: virtually all that did not escape ends up in a
superheated atmosphere. On cooling, much of the remaining H2O
and some of the CO2 redissolves in the MO. Upon MO
crystallization, much of the H2O and possibly nearly all the the C
returns atmosphere. However, homogenization of Fe3+/FeT in the
MO should again produce a vertical fO2 gradient in the MO and the
solubility of CO2 diminishes by >10X on cooling from a MO adiabat
to the peridotite solidus. Thus, precipitation of a C-rich phase
(diamond, carbide, or alloy) during crystallization of this final MO is
likely, setting up conditions for a MO carbon pump [5] that draws
down much of the CO2 in the atmosphere and deposits much of the
BSE C in the mantle rather than the exosphere. This speculative, but
plausible, scenario may produce a mantle with a high H/C ratio
compared to the exposphere, as remains true at present [6].
[1] Kress&Carmichael (1991) Contrib. Mineral. Petrol. 108, 82-92.
[2] O’Neill et al. (2006) Am. Mineral 91, 404-412.[3] Dahl &
Stevenson (2010) EPSL 295 177-186. [4] Suzuki et al. (1995)
Science 269 216-281. [5] Hirschmann (2011) 42nd LPSC Abstracts
#2321. [5] Hirschmann&Dasgupta (2009) Chem. Geol. 262 4-16.
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